In the machining of valve seats, particularly in internal combustion engines, the highest precision of the valve seat is required. Valve seats are machined by turning, whereby the shaping tool serving as the rotary tool is guided on a pilot shaft, which serves as a guide mandrel in the valve guide of the valve for the valve seat to be machined. The snug fitting of the valve attained through this machining depends directly upon the snug fitting or position of the pilot shaft in the valve guide. Even with a nearly perfectly snug fitting of the pilot shaft, it is unavoidable that this has some clearance in the valve guide, even if only to a slight degree. This is particularly the case if the valve guide is somewhat deflected, and is thus extended in its diameter toward its ends. The present invention concerns a process and a device by means of which such a pilot shaft can be optimally centered in a valve guide, so that any possible clearance of the pilot shaft in the valve guide is averaged out.
Valve seats are conventionally machined with devices which have a support stand which is mounted on a magnetic flange. The magnetic flange has a flat support surface by means of which it can be set on a clamping plate, and can be fixed there in an immovable manner by means of magnetic forces. The support stand comprises a support column, on which a swivel arm is positioned in a height-adjustable manner. The swivel arm bears on its one end a support mount for a swivel ball, in which a pilot shaft intended to act as a guide mandrel is inserted. The pilot shaft can thereby be swiveled on all sides relative to the support mount, and can be fixed by means of the support mount in any chosen swivel position, which mount is for this purpose provided with a clamping device. A shaping tool is supported on the pilot shaft as a rotary tool. The turning of the rotary tool can take place by hand, but also with the aid of mechanical, pneumatic, hydraulic or electrical drive devices.
The device is now employed in accordance with a process in which the pilot shaft, in the detached condition of the swivel ball, is inserted into the valve guide. In this manner, the most closely fitting possible pilot shaft, which should have the least possible clearance in the valve guide, is used. After inserting the pilot shaft, the swiveling ball is clamped solidly in its support mount, and the support stand is then, by turning on the magnetic field of the magnetic flange, attached in an immovable manner to the clamping plate. With the rotating shaping tool, it is now smoothly brought down to the valve seat, and as much material is removed by turning as needed, until the shaping tool has evenly shaven away the valve seat material. It is, of course, to be understood that the pilot shaft in this process unavoidably has a certain clearance in the valve guide, within which it has a certain imprecision relative to its exact central position. If the valve guide is now additionally deflected, this clearance increases. Even if this only involves a few fractions of an angle of degree, such an imprecision of the pilot shaft, which should serve as the guide for the shaping tool, has a considerable effect on the later snug fitting of the valve in the valve seat to be machined.
As an auxiliary measure, certain devices are equipped with an air-supported magnetic flange, so that, upon inserting the pilot shaft, the frictional forces between the magnetic flange and the clamping plate are minimized. The magnetic flange hovering on an air cushion makes it possible for the pilot shaft in the valve guide to be able to occupy as stress-free a position as possible. If the magnetic flange at first no longer shifts on the clamping plate, it is fixed on the clamping plate by switching on the magnetic field. Even with this device, however, a certain clearance of the pilot shaft in the valve guide is unavoidable, particularly if the valve guide is still deflected. An optimal centering of the pilot shaft is thus unattainable by means of the conventional machining devices and the processes applied with them.